You should print out your return and compare it side by side, line by line to last year and see where the differences are. Maybe you'll spot something you left out or entered wrong.
It could just be that your withholding wasn't enough to cover the tax on your income. Or you owe Self Employment tax. Or you owe a penalty for something. Like the penalty for a IRA or 401k early withdrawal or the Health Care penalty for not having Health Insurance.
